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Lasers Can Lengthen Quantum Bit Memory By 1,000 TimesScienceDaily (June 25, 2009) Physicists have found a way to drasticallyprolong the shelf life of quantum bits, the 0s and 1s of quantum computers.

These precarious bits, formed in this case by arrays of semiconductor quantum dots

containing a single extra electron, are easily perturbed by magnetic field fluctuations from the

nuclei of the atoms creating the quantum dot. This perturbation causes the bits to essentially

forget the piece of information they were tasked with storing.

A quantum dot is a semiconductor nanostructure that is one candidate for creating quantum

bits. The scientists, including the University of Michigan's Duncan Steel, used lasers to elicit

a previously undiscovered natural feedback reaction that stabilizes the quantum dot's

magnetic field, lengthening the stable existence of the quantum bit by several orders of

magnitude, or more than 1,000 times.

Because of their ability to represent multiple states simultaneously, quantum computers could

theoretically factor numbers dramatically faster and with smaller computers than

conventional computers. For this reason, they could vastly improve computer security.

"In our approach, the quantum bit for information storage is an electron spin confined to a

single dot in a semiconductor like indium arsenide. Rather than representing a 0 or a 1 as a

transistor does in a classical computer, a quantum bit can be a linear combination of 0 and 1.

It's sort of like hitting two piano keys at the same time," said Steel, a professor in the

Department of Physics and the Robert J. Hiller Professor of Electrical Engineering and

Computer Science.

"One of the serious problems in quantum computing is that anything that disturbs the phase

of one of these spins relative to the other causes a loss of coherence and destroys the

information that was stored. It is as though one of the two notes on the piano is silenced,

leaving only the other note."

Spin is an intrinsic property of the electron that isn't rotation, but is more like magnetic poles.

Electrons are said to have spin up or down, which represent the 0s and 1s.

A major cause of information loss in a popular class of semiconductors called 3/5 materials is

the interaction of the electron (the quantum bit) with the nuclei of the atoms in the quantum

dot holding the electron. Trapping the electron in a particular spin, as is necessary in quantumcomputers, gives rise to a small magnetic field that couples with the magnetic field in thenuclei and breaks down the memory in a few billionths of a second.

By exciting the quantum dot with a laser, the scientists were able to block the interaction of

these magnetic fields. The laser causes an electron in the quantum dot to jump to a higher

energy level, leaving behind a charged hole in the electron cloud. This hole, or space vacated

by an electron, also has a magnetic field due to the collective spin of the remaining electron

cloud. It turns out that the hole acts directly with the nuclei and controls its magnetic field

without any intervention from outside except the fixed excitation by the lasers to create the

hole.

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"This discovery was quite unexpected," Steel said. "Naturally occurring, nonlinear feedback

in physical systems is rarely observed. We found a remarkable piece of physics in nature. We

still have other major technical obstacles, but our work shows that one of the major hurdles to

quantum computers that we thought might be a show-stopper isn't one," Steel said.

Other authors are with the Naval Research Laboratory, the University of California San

Diego, and the University of Hong Kong. The research is funded by the U.S. Army ResearchOffice, the Air Force Office of Scientific Research, the Office of Naval Research, The

National Security Agency's Laboratory for Physical Sciences, the Intelligence Advanced

Research Projects Agency and the National Science Foundation.